The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. As illustrated in FIGS. 1A and 1B, a conventional combustible gas sensor 10 typically includes an element such as a platinum element wire or coil 20 encased in a refractory (for example, alumina) bead 30, which is impregnated with a catalyst to form an active or sensing element, which is sometimes referred to as a pelement 40, pellistor, detector or sensing element. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987). Combustible gas sensors are also discussed generally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases, Heinemann, Exeter, 29 (1981).
Bead 30 will react to phenomena other than catalytic oxidation that can change its output (i.e., anything that changes the energy balance on the bead) and thereby create errors in the measurement of combustible gas concentration. Among these phenomena are changes in ambient temperature, humidity and pressure.
To minimize the impact of secondary effects on sensor output, the rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of sensing element or pelement 40 relative to a reference resistance embodied in an inactive, compensating element or pelement 50. The two resistances are typically part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG. 1B. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement 50 are typically matched as closely as possible with active or sensing pelement 40. Compensating pelement 50, however, typically either carries no catalyst or carries an inactivated/poisoned catalyst.
Active or sensing pelement 40 and compensating pelement 50 can, for example, be deployed within wells 60a and 60b of an explosion-proof housing 70 and can be separated from the surrounding environment by a flashback arrestor, for example, a porous metal frit 80. Porous metal frit 80 allows ambient gases to pass into housing 70 but prevents ignition of flammable gas in the surrounding environment by the hot elements. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
The operation of a catalytic combustible gas sensor proceeds through electrical detection of the heat of reaction of a combustible gas on the oxidation catalyst, usually through a resistance change via a Wheatstone bridge as described above. The oxidation catalysts may, for example, operate in the temperature range of 350-600° C. for methane detection. The sensor must sufficiently heat the pelement through resistive heating. Generally the heating and detecting element (element 20) are one and the same. A platinum alloy is often used because of its large temperature coefficient of resistance, resulting in a large signal in target or analyte gas.
As described above, the heating element may be a helical coil of fine wire. The heating element can also be a planar meander formed into a hotplate or other similar physical form. The catalyst being heated often includes an active metal catalyst dispersed upon a refractory catalyst substrate. Usually the active metal is one or more noble metals such as palladium, platinum, rhodium, silver, and the like and the refractory metal oxide support consists of one or more oxides of aluminum, zirconium, titanium, silicon, cerium, tin, lanthanum and the like, which may or may not have high surface area greater than 75 m2/g. The support and catalytic metal precursor may be adhered to the heating element in one step or in separate steps using thick film or ceramic slurry techniques as known in the art. Often, a catalytic metal salt precursor is heated during manufacture to decompose it to the desired dispersed active metal, metal alloy, and/or metal oxide.
Oxidation catalysts formed onto a helical wire heater are typically referred to as pelements while those formed onto hotplates (whether microelectronic mechanical systems (MEMS) hotplates or conventional, larger hotplates) are sometimes known by the substrate. Oxidative catalysts formed on MEMS heating elements are sometimes referred to herein as MEMS pellistors. As described above, the detecting pelements or catalytically active hotplates can be paired with a similarly sized heater coated with materials with similar thermal conductivity as the active catalyst but without active sites. The inactive pelement or hotplate may be used to compensate for changes in ambient temperature, relative humidity, or background thermal conductivity not associated with a combustible gas and are therefore often referred to as compensators. The matched pair of detecting and compensating elements can be assembled in a Wheatstone bridge configuration for operation and combustible gas detection, which requires that both the detector and compensator operate at the same elevated temperature. Alternately, the compensator function can be achieved by using a detecting pelement or hotplate that is operated well below the minimum oxidation temperature using an electronically controlled independent bridge circuit as taught in U.S. Pat. No. 8,826,721. Advantages of the independent bridge circuit operating mode include power savings and longer life due to switching active detector pelements or hotplates.
It is well known that oxidation catalysts can suffer deactivation as a result of catalyst poisons and inhibitors such as compounds containing silicone, sulfur, phosphorus, and lead which make their way to the catalyst from the gas phase but become bound to the solid catalyst surface. To ameliorate the effects of environmental poisons, catalytic combustible sensors may include filtration material(s) upstream of the active catalyst to trap inhibiting or poisoning compounds. Such filters may, for example, rely on the actions of physisorption, chemisorption, chemical reaction, or a combination thereof to increase the span, stability and lifetime of the combustible sensor. Filters may, for example, include a variety of metal salts, activated carbon, adsorbent metal oxides or combinations thereof which have been found to reduce the effective concentration of poisons reaching the catalyst. See, for example, U.S. Pat. No. 6,756,016. A consequence of adding upstream filtration to combustible sensors is that span and response time may be reduced for combustible gases of interest (particularly, for heavy hydrocarbons when adsorbents with a surface area greater than 75 m2/g are employed or thick filters are used). Upstream filtration is not limited to external filters and may include materials coated directly onto the catalyst surface.